Bright zinc plating



baths and chloride baths.

Patented Oct. 12, 1948 I nnron'r'zmo PLA'mIG Robert R. Hair and Barry L.Bonner, Niagara Falls, and Joseph Frederic Walker, Lewiston, N. -Y.,asslgnors to E. I. du Pont de Nemours It Company, Wilmington, Del., acorporation of Delaware No Drawing. Application November 25,1948,

Serial No. 511,686

This invention relates to zinc plating and, more particularly, to theproduction of bright zinc electrodeposits.

A number of processes have been developed 4 for plating bright zinc fromcyanide baths by means of which excellent bright deposits can beobtained, provided the proper conditions such 4 Claims. (Cl. 201-55)""1; polyvinyl alcohol addition agent, excellent bright deposits can beobtained over a wide range of current densities at temperatures from C.

to as high as 60 C. and even higher. In other 'words, the addition ofthe condensation product widens-the current density bright range at theas temperature andcurrent density are observed.

An example oi a successful bright zinc plating process is that describedand claimed in U. 8. Patent 2,171,842 wherein the bright deposit isobtained by the addition of a small amount of polyvinyl alcohol to theplating bath. This proc-, ess. produces very bright deposits of zinc andover a wide current density range at low temperatures such as to C.However, it is often desired to operate such a bath at highertemperatures, such as 50 to 60 0., whereby more rapid electroplating maybe obtained. At such high temperatures the polyvinyl alcohol stillfunchigher temperatures.

The aforesaid condensation product is described and claimed in acopending patent application, Serial No. 507,182, filed October 21,1943' by Joseph F. Walker, now Patent No. 2,411,396. This condensationproduct may be made by the addition 1 sodium cyanamide to aqueousformaldehyde so utions, the reaction being-carried out under alkalineconditions at a temperature below 0., preferably inthe range of 15to 20C. The reaction between the cyantions as an excellent brightening agent,but the current density bright range at such high temperatures ismaterially limited. It is desirable, therefore, to have a brighteningagent which will produce the bright deposits obtainable by means ofpolyvinyl alcohol and which can be employed for that purpose at elevatedtemperatures and over a wide current density range at such elevatedtemperatures. 3

Addition aBents are often used heretofore in various acid zinc platingbaths such as sulfate The effect oi such additions is to whiten theelectrodeposit, but heretofore it has not been possible to producelustrous, bright zinc plate from acid baths.

An object oi the present invention is to provide an improved process forelectroplating bright zinc deposits. I A further object is to provide anaddition agent for bright zinc plating baths which is operable over awide range oi temperatures and current density, and which is eiiectivein alkaline, acidic and neutral zincplating solutions.

A further object is to provide an electroplating bath addition agentcomposition as hereinafter set forth. from the following description ofour invention.

We have discovered that a certain polymeric condensation productoisodium cyanamide-and formaldehyde, which is soluble in alkalinesolutions having a pH above 9 and insoluble in water at a pH oi B to 9,is an excellent brightening agent for zinc plating baths, whetheralkaline, acidic or neutral. We have further discovered that when thiscondensation product is added to an alkaline zinc plating bathwhichalready contains Still other objects will be apparent amide and theformaldehyde solution is exothermic and cooling is generally necessaryto maintain the temperature below 35 C. After the reaction has beencompleted, the condensation product is precipitated from the reactionmixture by adding sumclent acid to reduce the alkalinity of the solutionto a pH within the range 8 to 9. It may then be filtered ed and dried,the pure product being a white powder.

The product is probably a partially hydrated polymer of methylenecyanamide (CI-Ia=NCN) in which approximately two-thirds of the ONradicals have been converted to amide groups.

Analyses for carbon, hydrogen, .and nitrogen indicate that itscomposition closely approximates.

the empirical formula, CsHmNoOn. The characteristics of the product arethose of an amorphous resin, and it is probably not a simple compoundoilow molecular weight but rather a polymer or mixture of polymers made upof units corresponding to the CsHioNsO: formula. It is believed thatthestructural iormula oi this polymer unit is as follows:

.The new condensation product is a white, apparently non-crystallinesolid which chars when heated to temperatures above 300 C. Although itis apparently stable when heated below 300 C. when dry, it undergoesfurther resiniilcation. when heated in the moist state and should bedried'quickly and at relatively low temperatures to avoid undesirablealteration. On long heating in the moist state-it combines withadditional water and tends to liberate ammonia. I A sample heated inthis way was found to have an embeing in the form of dry powders.

pirical composition corresponding to Cs'HnNsOs.

The new product dissolves readily in dilute aqueous acids or strongalkalies and is relatively insoluble in pure water, the product beingleast soluble in aqueous liquids having a pH of 8.0-9.0. when dissolvedin aqueous alkalies or acids it may be precipitated therefrom, althoughthe yields upon reprecipitation are not entirely quantitative. Analysesof the reprecipitated product indicate that its composition issubstantially unchanged. v q I In using this condensation product as abrightvening agent, the amount to be added to the bath will vary uponthe degree of brightening desired to be obtained. Generally. it isnecessary to use at least about 0.1 ounce per gallon in order to obtainappreciable brightening effect. In amounts up to 2 ounces per gallon ofthe conden-.

- sation product, excellent brightening is obtained over a wide range ofcurrent densities. Within this range generally the best results areobtained with a concentration of about 0.5 ounce per gallon. Atconcentrations above 2 ounces per gallon the brightening eflect tends tooccur only at relatively high cathode current density such as 50to 500amps. per sq. ft. Thus, when recessed articles are electroplated withsuch high toncentration of the brightening agent, the areas of thecathodeclosest to the anode will plate bright, whereas the recesseswhich have a low current density tend to plate dull. With thislimitation. our new brightening agent may 'be used in any concentrationup to the limit ofits solubility. which is approximately 16 ounces pergallon. If more of the addition agent isv added than can dissolve in theplating bath, it is desirable to filter Example 1 A zinc plating bathwas made by dissolvingthe following ingredients in water Ounces pergallon Zinc cyanide v 8.0 Sodium cyanide 5.59

' Caustic soda 10.0

The solution was purified was then operated to amps. per sq. ft. at atemperature of 25 C. with pure zinc anodes. The resultingelectrodeposit. after bright dipping in dilute nitric acid, 'was whitebut not bright. 20 grams per gallon of the condensation prodnot ofsodium cyanamide and formaldehyde was then added to the solution andwhen thishad dissolved, the solution was again operated at 40 amps. persq. it. at 25 C. The electrodeposit after bright dipping had excellentbrightness.

Example 2 Apurifled zinc plating solution was prepared as in Example 1and to this was added 0.3 gram per gallon of polyvinyl alcohol inaqueous solution and 2.25 grams per gallon of the above-described posithad a hazy white color. At current denslties from 6.5 to 100 amps. persq. ft. excellent "bright plate was obtained. The bath was operated at atemperature of 80 C.-

the bath to remove excess undissolved material.

the presence of which will tend to cause roughness in the cathodedeposit. 1

In practicing our invention by the conjoint addition of polyvinylalcohol and the condensation product to an alkaline zinc plating bath,the amounts oi polyvinyl alcohol suitable for zinc plating baths may beused for example, the amounts shown in U. S. Patent'2,171,842. Theoptimum amount of polyvinyl alcohol will vary,

and in most cases will lie in the range of 0.001 to r 0.1 ounce pergallon. Generally we prefer to add not more than about 0.03 ounce pergallon of polyvinyl alcohol, e. g., 0.001 to 0.03 ounce per gallon inplating baths like those shown in the examples given hereinafter.

In a preferred method for practicing our invention. we prepare a mixtureof polyvinyl alcohol and the condensation product containing 1 to 5% byweight of the polyvinyl alcohol. both substances A conven: tional zinccyanide plating bath is then prepared by dissolving in water, zinccyanide. sodium cyanide and caustic soda and this solution is purifiedso as to remove heavy metal ions. for example. by treatment with sodiumsulfide or metallic zinc powder. followed by filtration if necessary.About V. ounce per gallon of the abovedescribed mixture of polyvinylalcohol and condensation product is then dissolved in the plating bath.If desired, the dry addition agent may first be dissolved in watercontaining suflicient caustic soda to give a pH of 10 to 12. Theresulting electroplating bath maythen be operated in the conventionalmanner at any desired temperature, e. g. 25 to 60 C. and at cathodecurrent densities up to 300 amps. per sq. ft.

The following examples further illustrate O invention:

Example 3 To a zinc plating solution prepared as in Example 1, 4 gramsper gallon of polyvinyl alcohol and 15 "grams per gallon of thecondensation. product was added. With a bath temperature of 50 C;cathodic deposits were obtained at current densities from 20 to 180amps. per sq. it. which had excellent brightness after bright dipping inExample 4 The following zinc cyanide plating solution was made'up:

Ounces per gallon Zinc cyanide 8.0 Sodium cyanide 3.4 Caustic soda 710.0

This was purified by addition of ,4 ounce per gallon of sodium sulfideand ounce per gallon of a mixture of powdered polyvinyl alcohol,containing 3% by weight of polyvinyl alcohol, was dissolved-in the bath.This bath then was utilized to electroplate strip steel at cathodecurrent densities of 200 to 300 amps. per sq.-ft. After bright dipping,the resulting electrodeposits were bright and pleasing in appearance.The cathode current efiiciencies were 90.5% at 200 amps. per sq. ft. and69.5% at 300 amps. per sq. ft.

L's-ample 5 To the zinc plating solution of Example 1 was.

added 9 grams per gallon of a mixture of equal parts by weight of theaforesaid condensation product and anisaldehyde. Excellent brightdeobtained at current densities of 3 to ampspel sq. ft. With this bathgood bright by the addition of As ounce per gallon of sodium suliide.The bath the condensation product and deposits could be obtained withoutbright dipp Example 6 I The method of Example 5 was repeated except thatthe zinc plating solution had the following formula:

Ounces per gallon Zinc cyanide Q. 8.0 Sodium cyanide v 4.49 Caustic,soda 10.0

Example 7' The following acidic zinc plating solution was prepared:

Zinc chloride -ounces per-gallon..- Ammonium chloride do 28.0 AlCh ..dopH adjusted to elcctrometric 3.5

The above solution, operated at 25 0., plated a white, crystalline,non-lustrous zinc deposit at current densities up to 150 amps. per sq.it. An

X 80' O. The deposits were dark gray at current densities of to 25 amps.per sq. ft. and spongy and non-adherent at higher current ensities. Theaddition of 1 ounce per gallon of t e mixed addition agent of Example 8gave pleasing white,

lustrous zinc deposits at current densities of 10 to 60 amps. per sq.ft.

addition of 0.5 ounce per gallon of the hereindescribed condensationproduct gave an'excellent bright zinc deposit at current densities ofabout 40 to 125 amps. per sq. ft.

' Example 8 A sodium zincate bath was prepared as follows:

Ounces per gallon Zinc hydroxide 7.5 Caustic soda 20.0

This solution, operated at 0., produced a white, crystalline.non-lustrous zinc plate at current densities of 4 to 10 amps. per sq.ft. At higher current densities, the deposits were gray and spongy. Anaddition of 1 ounce per gallon of a mixture of 97 parts by weight of thecondensation product and 3 parts by weight of polyvinyl alcohol gave asemi-bright, lustrous deposit at current densities of 10 to 100 amps.per sq. ft.

Without addition agent deposits plated from this .solution at a bathtemperature of 25 C. were gray to white matte and at current densitiesin excess of. approximately amps. per sq. ft.. the deposits were darkgray. The addition of 0.5 ounce per gallon of the condensation, productgave brilliant, lustrous deposits at current densities of 10 to above125 amps. per sq. ft.

Example 10 A zinc pyrophosphate plating bath was prepared as follows:

Zinc pyrophosphate ounces per gallon.. 8.0 Sodium sulfate do.. 4.0 NH4OHto a pH of -electrometric.. 11.5

This bath was operated at a bath temperature of Our invention alsoincludes the novel zinc plating addition agent which comprises a mixtureof the condensation product and polyvinyl alcohol in suitableproportions. The preferred composition is made by mixing the solid,pulverulent condensation product with anhydrous, finely dividedpolyvinyl alcohol so that the mixture contains 1 to 5% by weight ofthepolyvinyl alcohol. This may be dissolved in caustic soda solution orother aqueous alkaline liquid to prepare a. stock solution. We havefound the two ingredients to be mutually compatible, and substantiallynon-reactive towards each other, both in the anhydrous state and insolution. If desired, other substances may be added to the mixture,either in solid or in solution form, for example, plating bathingredients, other addition agents, dyes or pigments, or inertmaterials. Excellent results have been obtained with a pulverulentmixture containing approximately 3% by weight of polyvinyl alcohol and97% by weight of the condensation product.

As shown by the foregoing examples, our novel brightening agent may beused in zinc plating baths either alone or in combination with otherbrightening agents, including both organic agents and metallic compoundssuch as salts of molybdenum, manganese, chromium and the like. As thecondensation product has a low solubility at a pH of 8 to 9, it ispreferable that the plating bath have a pH below 8 or higher than 9,This presents no difilculty' as alkaline zinc cyanide plating bathsusually have the required high pH and the acid to neutral baths have apH below 8. While we prefer to utilize our addition agent in finecyanide baths, which have been purified to remove heavy metal ions, theinvention'is not restricted thereto as improved electrodepositsareobtained in baths which have not been so purified.

Likewise, the invention is not restricted to the bath formulas shown inthe above examples, which have been given merely by way of example. Ournovel brightening agent is effective for electroplating from any aqueoussolution containing a zinc compound in solution. In baths whichinherently produce inferior 'zinc deposits, the appearance of thedeposit is invariably improved by addition of our condensation product.Baths which inherently produce superior zinc deposits generally aremarkedly improved by meansof our addition agent and are preferred inpracticing our invention. Thus, to produce bright, lustrous zincdeposits according to our invention, we generally prefer to utilize thezinc chloride and zinc sulfate acid baths and the zinc cyanide alkalinebaths.

Suitable zinc cyanide baths may be made, for

example, by dissolving zinc cyanide, oxide or carbonate in a solution ofan alkali metal cyanide, While we prefer to use sodium or potassiumcyanide, the cyanides of other alkali metals,"

cesium, ribidium and lithium may be used if desired. The bath maycontain the ions of one or "several of the alkali metals in anydesiredproing processes, the electrodeposit obtained from a cyanide bathby our process may have more orless of a brownish or bronze-like filmwhich can readily be'removed by conventional bright dipping proceduresto obtain maximum brightness.-

Ir. some cases the film is so slight that bright dipping is notnecessary or desirable, and our invention is not restricted to the useof a bright dip ride or zinc sulfate solutions, we prefer to havepresent in the bath a small amount of an aluminumsalt, for example, 1 toounces per gallon of aluminum chloride or aluminum sulfate.

While we prefer to use the herein-described invention to electrodepositpure zinc, the invention is not restricted thereto. If desired. vari:ous metal salts may be included in the plating bath to brighten orotherwise modify the cathodic depositv and the electrodeposit then maycontain up to 10% by weight of such added metal. Thus, the invention maybe used to produce improved electrodeposits composed of 90% or more byweight of zinc. Representative examples of such other metalliccomponents which may be thus codepositedare molybdenum. chromium,

manganese, copper, silver. mercury and cadmium.

We claim:

1. The process of electrodepositing zinc from an aqueous zinc cyanideelectroplating solution having a pH above 9.0 which compriseselectrodepositing said zinc from said solution containing dissolvedtherein about 0.1 to 2.0 ounces per gallon of the condensation productof sodium cyanamide and formaldehyde obtained by reacting substantiallyequimolecular amounts of sodium cyanamide and formaldehyde in aqueousalkaline solution at a temperature below 35 C.

2. The process of electrodepositing zinc from an aqueous zinc cyanideelectroplating solution having a pH above 9.0 which compriseselectrodepositing said zinc from said solution containing dissolvedtherein about 0.1 to 2.0 ounces per gallon of the condensation productof sodium cyanamide and formaldehyde obtained by reacting substantiallyalkaline solution at a temperature below 35' 0.. and about 0.001 to 0.1ounce per gallon of polyvinyl alcohol.

3. An aqueous zinc electroplating solution containing the necessarycomponents. for the electrodepositlon of zinc, said solution having apH. above 9.0 and containing dissolved therein as a brightening agent,0.1 to 2.0 ounces per gallon of the condensation product of sodiumcyanamide and formaldehyde obtained by reacting subetantiallyequimolecular amounts of sodium cyanamide and formaldehyde in aqueousalkaline solution at a temperature below 35 0.

"4. An aqueous zinc electroplating solution containing the necessarycomponents forthe eleetrodeposltion of zinc, said solution having n. pl!above-9.0 and containing dissolved therein as a brightening agent, 0.1to 2.0 ounces per gallon of the condensation product of sodium cyanamideand formaldehyde obtained by reacting substantially equimolecularamounts of sodium cyanamids and formaldehyde in aqueous alkalinesolutionat a temperature below 35 C., and 0.001 to 0.1 1 olmceper gallon ofpolyvinyl alcohol.

ROBERT R. mm. nanny L. BENNER. JOSEPH rannmrc wanna.

ass-Ea" ENOES orrnn The following references are of record in the fileof this patent:

UNITED STATES m'rau'rs Metal 1ndustry, Nov'. 29,1940, page 443.

equimolecular amounts of sodium cyanamide and formaldehyde in aqueous-

